Wavelength Division Multiplexing (WDM) optical networks have traditionally been used for long haul point-to-point transmission. However, with the increasing demands on communication systems, WDM optical networks can also be used in smaller system configurations, such as local telephone or data networks, e.g. LANS, MANS, etc. Communication signals are transmitted over a limited geographic area to various nodes within a network.
These types of communication networks typically include a plurality of nodes. Each node can include one or more optical add/drop multiplexers configured to select a corresponding one or more information-bearing or payload channels from a received WDM signal. An example of an add/drop multiplexer used in this type of network is described in co-pending U.S. patent application entitled "Optical Add/Drop Multiplexer" filed on Oct. 23, 1997 having unofficial Ser. No. 08/956,807 and assigned to the assignee of the present invention (hereinafter referred to as "copending Optical Add/Drop Multiplexer Application"). The information contained in the dropped or selected channels is processed and the channels carrying new information are added to the WDM signal for transmission to other nodes in the network. A service channel, usually outside the information bearing or payload channel bandwidth, is included in this type of network for carrying system housekeeping information, for example network monitoring and control signals as well as telemetry information, to and/or from nodes in the network.
Network nodes may include multiple Optical Add/Drop Multiplexers (OADMs) or other devices to drop different payload channels. When multiple OADMs, for example, are present within the same node, the service channel is dropped at the first OADM within the node and is added again at the last OADM before exiting the node. If a fault occurs between the intra-node OADMs, the service channel is incapable of detecting this fault because the service channel signal does not traverse each intra-node optical path. In some circumstances, the service channel doesn't traverse each OADM, nor does it traverse transceivers associated with each OADM.
Accordingly, there is a need for a method and apparatus for providing a service channel to monitor/control intra-node optical paths within a communications network.